Critical Role of Surface Steps in the Alloying of Ge on Si(001)

Using low-energy electron microscopy, we show that intermixing of Ge on Si(001) during growth is enhanced on stepped surfaces and is hindered on terraces where step flow does not occur. On large terraces we have identified a dramatic and unanticipated structural rearrangement that facilitates intermixing: Pairs of steps spontaneously form and migrate over the surface, leaving alloyed regions in their wake. The driving force for step formation is the entropy gain associated with the enhanced intermixing of Ge.

Figure 1

Sequence of 2.3 eV dark-field LEEM images showing the nucleation and migration of two stripe features. One stripe (black arrow) emerges from the bottom of the field of view, and the other (white arrow) from the top. The stripes nucleated after one monolayer of Ge was deposited at $750°\mathrm{C}$. When the two stripes meet, they annihilate. The images are labeled by the time after the stripes nucleate. Scale bar, $2\mu \mathrm{m}$.

Figure 2

(a)–(d) LEEM images of $1\mathrm{ML}/\min $ Ge growth on Si(001). (a) First-layer islands (white) nucleate and (b) coalesce into a “mesa.” (c) Second-layer islands (black) nucleate on the mesa layer. (d) Stripes form at the center of the mesa and rapidly move out to the edge. (e) AFM image showing the detailed structure of the stripe. The image corresponds to the boxed area in (d). The stripes are an alternating chain of vacancy islands and adatom islands. Scale bar, $1\mu \mathrm{m}$. (f) Schematic of the stripe structure shown in (e). ML-high islands are white, ML-deep pits are dark grey.

Figure 3

(a) Side view of a surface step growing via step flow at times $t_3>t_2>t_1$. The grey scale schematically indicates the Ge concentration “buried” by the advancing step. (b) Side view of a terrace with a pit and an island. The graph shows the distribution of subsurface Ge after the pit has filled in and the island has grown.

Figure 4

LEEM images of Ge growth on a terrace with a ML-deep pit ($800°\mathrm{C}$). The ML-deep pit fills in during Ge deposition via step flow. The stripe does not pass through the area where the pit was located. Instead, the stripe forms a ring, surrounding the area were the concentration of subsurface Ge is largest. The dotted line in (f) indicates the size of the pit at the start of Ge growth. Scale bar, $3\mu \mathrm{m}$.